Myogenesis in the chick embryo has been extensively studied with respect to the mononucleated myoblast, the precursor to the multinucleated skeletal muscle fiber. Four distinct classes of myoblasts have been identified by clonal analysis. The appearance of one class has been shown to be dependent on the nervous system in vivo. A completely in vitro system for studying differentiative interactions between cultured neurons and chick embryo leg muscle cells will be used to identify the precursor to the nerve-dependent myoblast class. The nature of the precursor will be determined and its induced differentiation will be studied mechanistically. The developmental fates of the precursor and other muscle tissue cells will be determined and used to construct developmental cell lineages for myogenesis.